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<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0"
          name="viewport">
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    <meta content="width=device-width, initial-scale=1" name="viewport">
    <title>Document</title>
    <link href="../boot/css/bootstrap.min.css" rel="stylesheet">
    <!--核心jquery js-->
    <script src="../boot/js/jquery-3.4.1.min.js"></script>
    <!--bootstrap 核心 js-->
    <script src="../boot/js/bootstrap.min.js"></script>
    <style>
        @media (max-width: 1000px) {
            .aa {
                background-color: red;
            }

        }

        @media (min-width: 800px) {
            .bb {
                background-color: green;
            }
        }
    </style>
</head>
<body>
<div class="bb aa">
    this is xiaohei
</div>
</body>
</html>